滴状冷凝过程液滴自由表面温度场分析

The analysis of droplet surface temperature field during dropwise condensation process

对于滴状冷凝过程及其传热强化机理,一般通过分析冷凝壁面上液滴分布和运动规律进行研究,并且将单个液滴视为稳定的个体,很少涉及液滴内部运动特征.本文通过红外热像仪观测了纯蒸气滴状冷凝过程中,液滴运动时自由表面温度场的演化过程.发现在疏水壁面上,液滴由于合并或脱落而发生移动过程中,其自由表面温度先降低,而后升高并高于移动前温度.通过分析疏水表面上液滴移动过程的物理模型,认为液滴移动时表面液膜发生履带式滚动现象,或者发生液滴内部与自由表面附近的液体间形成对流和掺混现象.对液滴运动时表面温度演变规律的分析表明:触发液滴表面发生持续冷凝可能需要克服一个临界过冷度,当气液间温差超过该临界值时才诱发冷凝;液滴合并或脱落等整体运动过程,导致了液滴内部的运动特征,并促进了较大尺寸液滴表面发生直接冷凝,这为强化冷凝传热的研究提供新的思路.

The invistigations on dropwise condensation process and the mechanism of heat transfer enhancement are usually based on the droplet distribution and the movement principle of droplets on condensing surface.In the meanwhile,a single droplet is treated as a stable individual and the movement property inside the droplet is rarely considered.With infrared thermography,the surface temperature distribution of condensate droplet during steam dropwise condensation process is observed.The result shows that the temperature of droplet surface first decreases and then increases and up to a value higher than the initial one as the droplet migrates from one position to another.The droplet will roll and the surface film would be tracked when the droplet moves on the hydrophobic surface.With the convection inside the droplet,condensate near the wall moves to the surface side.The analysis of surface temperature evolution of droplet indicates that the continuous condensation on droplet surface may occur when the surface subcooling exceeds a critical value.The direct condensation on large droplet surface can be promoted by the dynamic process such as droplet coalescence or falling off,which provides a new approach to the condensation heat transfer enhancement.